Imaging lens

ABSTRACT

An imaging lens includes, sequentially from an object side, a first lens group having a positive refractive power; a second lens group having a negative refractive power; a third lens group having a positive refractive power; and a fourth lens group having a negative refractive power. The third lens group is configured by a single lens element, and is moved along an optical axis, whereby focusing is performed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an internal focus imaging lens having an anti-vibration function and suitable for digital cameras, video cameras, etc.

2. Description of the Related Art

Conventionally, numerous internal focus imaging lenses have been proposed that have an anti-vibration function of preventing image blur that occurs consequent to vibration. These imaging lenses further move a relatively light midsection lens group to perform focusing (see, for example, Japanese Patent Nos. 3745104, 3541283, and 3486541; Japanese Patent Application Laid-Open Publication Nos. 2008-145584 and 2005-321574).

Single lens reflex cameras are equipped with a mechanism that uses a mirror disposed just in front of the film, to reflect light that has passed through the imaging lens and thereby guide the light to the optical viewfinder to make the captured image and viewfinder image coincide. However, recently mirror-less single lens cameras that are significantly smaller in size and omit the reflective mirror that guides the image to the optical viewfinder have appeared. Originally, sub-reflective mirrors that guide the images to a distance measuring apparatus during auto-focusing were also disposed with these reflective mirrors. However, since mirror-less single lens cameras omit both the reflective mirror and the sub-reflective mirror, mainstream auto-focusing by phase difference detection cannot be performed in conventional single lens reflex cameras. Thus, in mirror-less single lens cameras, auto-focusing by contrast detection, which is mainstream among compact digital cameras and video cameras, is employed. However, in this case, with mainstream drive apparatuses such as DC motors and ultrasonic motors in conventional interchangeable lenses for single lens reflex cameras, necessary operations against wobble occurring with contrast detection auto-focusing are difficult. Consequently, drive apparatuses such as step motors used in compact digital cameras and video cameras are necessary.

Nonetheless, step motors have a low drive torque and with respect to a focusing lens group that is in a conventional optical system and that can be driven by a conventional DC motor or ultrasonic motor, numerous disadvantages arise related to the focusing lens group being too heavy to drive. With telephoto lenses, this tendency is strong. The same problems occur concerning the anti-vibration lens group. In particular, with a large diameter telephoto lens, since the lenses forming the anti-vibration lens group have a large diameter, anti-vibration lens group tends to be heavy. If the anti-vibration lens group becomes heavy, driving control at the time of vibration correction becomes difficult. In addition, if the lens diameter becomes large, the aberration that occurs also increases and consequently, to control the aberration, the number of lenses configuring the anti-vibration lens group has to be increased, making the anti-vibration lens group even heavier and creating a vicious circle.

The optical systems disclosed in the patent documents above are no exception and the driven focusing lens group and anti-vibration lens group are configured by multiple lenses and are thus, heavy, making driving control by a step motor, which has a low drive torque, difficult.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least solve the above problems in the conventional technologies.

An imaging lens according to one aspect of the present invention includes, sequentially from an object side, a first lens group having a positive refractive power; a second lens group having a negative refractive power; a third lens group having a positive refractive power; and a fourth lens group having a negative refractive power. The third lens group is configured by a single lens element, and is moved along an optical axis, whereby focusing is performed.

The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view (along an optical axis) of an imaging lens according to a first embodiment;

FIG. 2 is a diagram of various types of aberration (at infinity focus) in the imaging lens according to the first embodiment;

FIG. 3 is a cross sectional view (along the optical axis) of the imaging lens according to a second embodiment;

FIG. 4 is a diagram of various types of aberration (at infinity focus) in the imaging lens according to the second embodiment;

FIG. 5 is a cross sectional view (along the optical axis) of the imaging lens according to a third embodiment;

FIG. 6 is a diagram of various types of aberration (at infinity focus) in the imaging lens according to the third embodiment;

FIG. 7 is a cross sectional view (along the optical axis) of the imaging lens according to a fourth embodiment; and

FIG. 8 is a diagram of various types of aberration (at infinity focus) in the imaging lens according to the fourth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the accompanying drawings, exemplary embodiments of an imaging lens according to the present invention are explained in detail below.

The imaging lens according to the present invention includes sequentially from a side nearest an object (object side), a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a fourth lens group having a positive refractive power.

An object of the present invention is to provide a compact imaging lens having favorable imaging performance as well as a compact and light-weight focusing lens group and anti-vibration lens group. Thus, to achieve this object, the following conditions are set.

The imaging lens according to the present invention performs focusing by moving the third lens group along the optical axis. The third lens group is preferably configured by a single lens element to facilitate reductions in size and weight. In particular, configuration by a single lens is more preferable. A single lens element configuration of the third lens group, which is the focusing lens group, and reductions in size and weight enable the load on driving mechanism of the focusing lens group, power consumption and the outer diameter of the barrel to be reduced, and further enable a lens driving mechanism having a low drive torque (e.g., a step motor) to be employed. A single lens element may be a single ground lens, an aspheric lens, and a cemented aspheric lens; and does not include lenses that are not cemented to one another and have a gap therebetween such as 2 positive lenses.

Further, the imaging lens according to the present invention preferably satisfies the following conditional expressions, where f is the focal length of the entire optical system, f₁ is the focal length of the first lens group, f₂ is the focal length of the second lens group, f₃ is the focal length of the third lens group, and f₄ is the focal length of the fourth lens group. 0.4<f ₁ /f<0.5  (1) 0.2<|f ₂ |/f<0.4  (2) 0.4<f ₃ /f<0.6  (3) 2.0<|f ₄ |/f<16.0  (4)

Conditional expression (1) indicates a condition for suitably setting the positive refractive power of the first lens group. Below the lower limit of conditional expression (1), the positive refractive power of the first lens group becomes too strong and higher order spherical aberration easily occurs. When higher order spherical aberration occurs at the first lens group, correction of this aberration is difficult at another lens group. On the other hand, above the upper limit of conditional expression (1), the refractive power of the first lens group becomes too weak whereby the back focus increases, increasing the overall length of the optical system.

Conditional expression (2) indicates a condition for suitably setting the negative refractive power of second lens group. Below the lower limit of conditional expression (2), spherical aberration is overcorrected at the second lens group and as a result, favorable imaging performance for the optical system overall becomes difficult to maintain. On the other hand, above the upper limit of conditional expression (2), various types of aberration (particularly, spherical aberration) occurring at the first lens group, which has a positive refractive power, cannot be completely corrected at the second lens group, which has a negative refractive power, and thus, is undesirable.

Conditional expression (3) indicates a condition for suitably setting the positive refractive power of the third lens group, which is the focusing lens group. Below the lower limit of conditional expression (3), the number of lenses configuring the third lens group has to be increased in order to correct spherical aberration and coma flare. As a result, reductions in the size and weight of the focusing lens group become impossible. On the other hand, above the upper limit of conditional expression (3), the refractive power of the third lens group becomes too weak, the distance that the third lens group is moved to perform focusing increases, and imaging performance for close range imaging significantly deteriorates.

Conditional expression (4) indicates a condition for suitably setting the negative refractive power of the fourth lens group. Below the lower limit of conditional expression (4), the number of lenses configuring the fourth lens group has to be increased in order to correct spherical aberration and coma flare. As a result, reductions in the size of the optical system become difficult, thereby inviting decreases in optical transmission in the overall optical system, which is undesirable. On the other hand, above the upper limit of conditional expression (4), the back focus of the optical system becomes too long and the overall length of the optical system increases.

The imaging lens according to the present invention preferably satisfies the following conditional expression, where β₃ is the lateral magnification of the third lens group and β₄ is the lateral magnification of the fourth lens group. 1.5<β₄ ²−(β₃×β₄)²<2.5  (5)

Conditional expression (5) indicates a condition for suitably setting a combination of the lateral magnification of the third lens group, which is the focusing lens group, and the lateral magnification of the fourth lens group, which is disposed closer to the image than the third lens group. Satisfaction of conditional expression (5) enables the overall length of the optical system to be shortened and imaging performance to be improved. Below the lower limit of conditional expression (5), the distance that the third lens group is moved during focusing increases, thereby increasing the overall length of the optical system and inhibiting reductions in the size of the optical system. On the other hand, above the upper limit of conditional expression (5), aberration variation during focusing becomes prominent, and thus, is undesirable.

The imaging lens according to the present invention preferably satisfies the following conditional expression, where f₁₋₂ is combined focal length of the first lens group and the second lens group and f is the focal length of the entire optical system. 1.0<f ₁₋₂ /f<2.0  (6)

Conditional expression (6) indicates a condition for suitably setting a combination of the first lens group having a positive refractive power and the second lens group having a negative refractive power, the first and the second lens groups configuring a telephoto optical system. Below the lower limit of conditional expression (6), the optical convergence by the first lens group and the second lens group becomes strong, making the correction of various types of aberration difficult. On the other hand, above the upper limit of conditional expression (6), the configuration of the optical system approaches an a focal optical system (an optical system having a focal length at infinity) and the overall length of the optical system increases.

In the imaging lens according to the present invention, the first lens group preferably includes 1 or more negative lenses that satisfy the following conditional expression, where υ_(dA) is the Abbe number for the d-line of the negative lens. υ_(dA)>60  (7)

The negative lens satisfying conditional expression (7) can be formed of an affordable glass material and therefore, by disposing even 1 negative lens formed of an affordable glass material in the first lens group, the manufacturing cost of the optical system can be reduced by the amount saved consequent to the use of the glass material.

In the imaging lens according to the present invention, a positive lens that satisfies the following conditional expression, where υ_(dB) is the Abbe number for d-line of the positive lens, is preferably disposed closest to the image, among the lenses of the first lens group (i.e., farthest on the image side of the first group). υ_(dB)<35  (8)

By disposing farthest on the image side of the first lens group, a positive lens that satisfies conditional expression (8), the second lens group, which is disposed closer to the image than the first lens group, can be configured by 1 lens, enabling reductions in the size and weight of the optical system to be facilitated.

The imaging lens according to the present invention further has an anti-vibration correction function of correcting blur that occurs when the optical system is subject to vibration. For example, when the optical system is subject to vibration consequent to handshake, the anti-vibration lens group is moved in direction substantially perpendicular to the optical axis, whereby blur is corrected. In the imaging lens according to the present invention, the fourth lens group includes sequentially from the object side, a front group having a negative refractive power, an intermediate group having a negative refractive power, and a rear group having a positive refractive power. The intermediate group having a negative refractive power has a function of an anti-vibration lens group. In other words, the intermediate group is moved in a direction substantially perpendicular to the optical axis (shifting of lens center), whereby blur that occurs when the optical system is subject to vibration is corrected. The intermediate group is preferably configured by a single lens element to facilitate reductions in size and weight, and is more preferably configured by 1 lens. A single lens element configuration of the movable intermediate group and reductions in size and weight enable the load on the driving mechanism of the intermediate group to be decreased and power consumption to be reduced. Furthermore, a lens driving mechanism having a low drive torque (e.g., a step motor) can be employed. A single lens element is defined as described above.

The imaging lens according to the present invention preferably satisfies the following conditional expression, where f_(ASM) is the focal length of the intermediate group of the fourth lens group, f_(REAR) is the focal length of the rear group of the fourth lens group, and f is the focal length of the entire optical system. 0.1<|f _(ASM) |/f<0.2  (9) 0.2<f _(REAR) /f<0.3  (10)

Conditional expression (9) indicates a condition for suitably setting the negative refractive power of the intermediate group of the fourth lens group. Below the lower limit of conditional expression (9), the refractive power of the intermediate group of the fourth lens group becomes too strong, whereby the occurrence of spherical aberration and chromatic difference of magnification becomes prominent. Here, aberration that has become prominent is difficult to correct at another lens group. On the other hand, above the upper limit of conditional expression (9), the refractive power of the front group becomes too weak and the distance that the intermediate group, which is the anti-vibration lens group, is moved (shifting of lens center) increases, and consequently, the correction angle necessary for the anti-vibration function cannot be established.

Conditional expression (10) indicates a condition for suitably setting the positive refractive power of the rear group of the fourth lens group. Below the lower limit of conditional expression (10), overcorrected aberration occurs at the rear group of the fourth lens group and as a result, favorable imaging performance in the optical system overall becomes difficult to maintain. On the other hand, above the upper limit of conditional expression (10), various types of aberration occurring at the intermediate group in the fourth lens group and having a negative refractive power cannot be completely corrected.

The imaging lens according to the present invention preferably satisfies the following conditional expression, where β_(REAR) is the lateral magnification of the rear group of the fourth lens group and β_(ASM) is the lateral magnification of the intermediate group of the fourth lens group. −2.0<β_(REAR)−(β_(ASM)×β_(REAR))<−1.6  (11)

Conditional expression (11) indicates a condition for suitably setting a combination of the lateral magnification of the intermediate group, which is the anti-vibration lens group and in the fourth lens group, and the lateral magnification of the rear group disposed on the image side of the intermediate group. Below the lower limit of conditional expression (11), aberration variation increases at the time of anti-vibration correction and thus, is undesirable. On the other hand, above the upper limit of conditional expression (11), the intermediate group of the fourth lens group is moved (shifting of lens center) over a greater distance at the time of anti-vibration correction, which increases the diameter of the optical system, and thus, is undesirable.

In the imaging lens according to the present invention, the fourth lens group may be configured as follows. In other words, the fourth lens group includes sequentially from the object side, the front group having a negative refractive power and a rear group having a positive refractive power. The front group having a negative refractive power has a function as an anti-vibration lens group and is moved substantially perpendicular to the optical axis (shifting of lens center), whereby blur occurring when the optical system is subject to vibration, is corrected. The front group is preferably configured by a single lens element to facilitate reductions in size and weight, and is more preferably configured by 1 lens. A single lens element configuration of the movable front group and reductions in size and weight enable the load on the driving mechanism of the front group to be decreased and power consumption to be reduced. Furthermore, a lens driving mechanism having a low drive torque (e.g., a step motor) can be employed. A single lens element is defined as described above.

When the fourth lens group is configured by 2 lens groups, the imaging lens according to the present invention preferably satisfies the following conditional expression, where f_(ASF) is the focal length of the front group, f_(REAR) is the focal length of the rear group, and f is the focal length of the entire optical system. 0.1<|f _(ASF) |/f<0.2  (12) 0.2<f _(REAR) /f<0.3  (10)

Conditional expression (12) corresponds to conditional expression (9) described above and when values deviate from the prescribed upper and lower limits, the disadvantages that arise are identical to those in the case of conditional expression (9).

When the fourth lens group is configured by 2 lens groups, the imaging lens according to the present invention preferably satisfies the following conditional expression, where β_(REAR) is the lateral magnification of the rear group and β_(ASF) is the lateral magnification of the front group. −2.0<{β_(REAR)−(β_(ASF)×β_(REAR))}<−1.6  (13)

Conditional expression (13) corresponds to conditional expression (11) described above and when values deviate from the prescribed upper and lower limits, the disadvantages that arise are identical to those in the case of conditional expression (11).

As described, according to the present invention, a compact imaging lens having a compact, light-weight focusing lens group and anti-vibration as well as favorable imaging performance can be realized. In particular, configuration of the focusing lens group and the anti-vibration lens group by a single lens element, respectively, enables further reductions in the size and weight of these movable lens groups. Consequently, the imaging lens is suitable for an imaging apparatus equipped with a lens drive apparatus such as a step motor having a low drive torque and can be used in an imaging apparatus equipped with a contrast detection autofocus mechanism. Further, satisfaction of the conditional expressions above suppresses the distance that the focusing lens group and anti-vibration, which are movable lens groups, are moved, thereby facilitating further reductions in the size of the optical system and enabling further improvement of the imaging performance of the optical system.

Hereinafter, with reference to the accompanying drawings, embodiments of the imaging lens according to the present invention will be described in detail. However, the present invention is not limited to the embodiments below.

FIG. 1 is a cross sectional view (along the optical axis) of the imaging lens according to a first embodiment. The imaging lens includes sequentially from a side (object side) nearest a non-depicted object, a first lens group G₁₁ having a positive refractive power, a second lens group G₁₂ having a negative refractive power, a third lens group G₁₃ having a positive refractive power, and a fourth lens group G₁₄ having a negative refractive power. An iris diaphragm STO is disposed between the first lens group G₁₁ and the second lens group G₁₂. The iris diaphragm STO has multiple diaphragm blades and by arbitrarily changing the aperture, controls the incident optical ray bundle and adjusts the F number. Between the fourth lens group G₁₄ and an image plane IMG, a filter FL, a field stop FS, a filter FL, and a cover glass CG are disposed sequentially from the object side. The filters FL are disposed to block infrared light. The field stop FS is disposed to suppress optical ray bundles that are not related to the image and cause unnecessary internal reflection. The cover glass CG is disposed to protect the image plane IMG. The filters FL and the cover glass CG are disposed as needed and when not necessary, can be omitted. The optical receiving surface of an image sensor, such as a CCD, CMOS, etc. is disposed at the image plane IMG.

The first lens group G₁₁ includes, sequentially from the object side, a negative lens L₁₁₁, a positive lens L₁₁₂, a positive lens L₁₁₃, a negative lens L₁₁₄, a positive lens L₁₁₅, a negative lens L₁₁₆, and a positive lens L₁₁₇. The positive lens L₁₁₅ and the negative lens L₁₁₆ are cemented.

The second lens group G₁₂ is configured by a negative lens L₁₂₁.

The third lens group G₁₃ is configured by a positive lens L₁₃₁. The surface of the object side of the positive lens L₁₃₁ is aspheric. The third lens group G₁₃ is moved along the optical axis, from the image plane IMG side toward the object side, whereby focusing from a state of infinity focus to a focus state for the minimum object distance is performed.

The fourth lens group G₁₄ includes, sequentially from the object side, a front group G_(14F) having a negative refractive power, an intermediate group G_(14M) having a negative refractive power, an aperture stop AS, and a rear group G_(14R) having a positive refractive power. The aperture stop AS suppresses the occurrence of aberration, which causes deterioration of the imaging performance of the optical system. The front group G_(14F) includes, sequentially from the object side, a negative lens L₁₄₁ and a positive lens L₁₄₂. The negative lens L₁₄₁ and the positive lens L₁₄₂ are cemented. The intermediate group G_(14M) is configured by a negative lens L₁₄₃. Both surfaces of the negative lens L₁₄₃ are aspheric. The intermediate group G_(14M) has a function as the anti-vibration lens group. In other words, the intermediate group G_(14M) is moved in a direction that is substantially perpendicular to the optical axis (shifting of lens center), whereby blur that occurs when the optical system is subject to vibration consequent to handshake, is corrected. The rear group G_(14R) includes, sequentially from the object side, a negative lens L₁₄₄, a positive lens L₁₄₅, a positive lens L₁₄₆, and a negative lens L₁₄₇. The negative lens L₁₄₄ and the positive lens L₁₄₅ are cemented.

Various values related to the imaging lens according to the first embodiment are indicated below.

(Lens data) r₁ = 209.873 d₁ = 5.000 nd₁ = 1.51680 νd₁ = 64.2 r₂ = 130.945 d₂ = 0.500 r₃ = 100.587 d₃ = 19.707 nd₂ = 1.49700 νd₂ = 81.6 r₄ = −407.748 d₄ = 0.300 r₅ = 127.811 d₅ = 11.535 nd₃ = 1.49700 νd₃ = 81.6 r₆ = −3470.036 d₆ = 2.700 r₇ = −372.417 d₇ = 4.000 nd₄ = 1.51680 νd₄ = 64.2 r₈ = 67.774 d₈ = 3.449 r₉ = 85.849 d₉ = 15.763 nd₅ = 1.49700 νd₅ = 81.6 r₁₀ = −303.004 d₁₀ = 4.000 nd₆ = 1.80610 νd₆ = 33.3 r₁₁ = 303.004 d₁₁ = 0.200 r₁₂ = 102.623 d₁₂ = 7.114 nd₇ = 1.84666 νd₇ = 23.8 r₁₃ = 406. 606 d₁₃ = 21.677 r₁₄ = ∞ (iris diaphragm) d₁₄ = 12.652 r₁₅ = 345.746 d₁₅ = 2.000 nd₈ = 1.84666 νd₈ = 23.8 r₁₆ = 66.823 d₁₆ = 29.779 r₁₇ = 92.929 (aspheric surface) d₁₇ = 4.000 nd₉ = 1.58313 νd₉ = 59.5 r₁₈ = 1608.056 d₁₈ = 5.000 r₁₉ = 84.455 d₁₉ = 1.700 nd₁₀ = 1.90366 νd₁₀ = 31.3 r₂₀ = 29.625 d₂₀ = 6.684 nd₁₁ = 1.48749 νd₁₁ = 70.4 r₂₁ = −416.181 d₂₁ = 4.395 r₂₂ = −81.133 (aspheric surface) d₂₂ = 2.000 nd₁₂ = 1.58313 νd₁₂ = 59.5 r₂₃ = 45.256 (aspheric surface) d₂₃ = 5.783 r₂₄ = ∞ (aperture stop) d₂₄ = 8.748 r₂₅ = 120.487 d₂₅ = 1.900 nd₁₃ = 1.84666 νd₁₃ = 23.8 r₂₆ = 36.921 d₂₆ = 8.887 nd₁₄ = 1.80610 νd₁₄ = 33.3 r₂₇ = −161.596 d₂₇ = 16.221 r₂₈ = 258.217 d₂₈ = 7.846 nd₁₅ = 1.80518 νd₁₅ = 25.5 r₂₉ = −66.637 d₂₉ = 6.629 r₃₀ = −72.972 d₃₀ = 1.600 nd₁₆ = 1.80420 νd₁₆ = 46.5 r₃₁ = 916.012 d₃₁ = 8.232 r₃₂ = ∞ d₃₂ = 2.000 nd₁₇ = 1.51680 νd₁₇ = 64.2 r₃₃ = ∞ d₃₃ = 8.000 r₃₄ = ∞ (field stop) d₃₄ = 39.800 r₃₅ = ∞ d₃₅ = 2.200 nd₁₈ = 1.51680 νd₁₈ = 64.2 r₃₆ = ∞ d₃₆ = 1.000 r₃₇ = ∞ d₃₇ = 1.000 nd₁₉ = 1.51680 νd₁₉ = 64.2 r₃₈ = ∞ d₃₈ = 1.000 r₃₉ = ∞ (image plane) (Constants of the cone (k) and aspheric coefficients (A₄, A₆)) (Seventeenth plane) k = −3.34693, A₄ = 5.79748 × 10⁻⁷, A₆ = 2.89688 × 10⁻¹¹ (Twenty-second plane) k = −1.89878, A₄ = 0, A₆ = 0 (Twenty-third plane) k = −1.77787, A₄ = 0, A₆ = 0 f (focal length of entire optical system) = 294.00 F no. = 2.88 2ω (angle of view) = 8.3 (Values related to conditional expression (1)) f₁/f = 0.444 (Values related to conditional expression (2)) |f₂|/f = 0.331 (Values related to conditional expression (3)) f₃/f = 0.572 (Values related to conditional expression (4)) |f₄|/f = 4.033 (Values related to conditional expression (5)) β₄ ² − (β₃ × β₄)² = 1.545 (Values related to conditional expression (6)) f₁₋₂/f = 1.154 (Values related to conditional expression (7)) ν_(dA) (Abbe number for d-line of negative lens L₁₁₁, negative lens L₁₁₄) = 64.2 (Values related to conditional expression (8)) ν_(dB) (Abbe number for d-line of positive lens L₁₁₇) = 23.8 (Values related to conditional expression (9)) |f_(ASM)|/f = 0.168 (Values related to conditional expression (10)) f_(REAR)/f = 0.244 (Values related to conditional expression (11)) β_(REAR) − (β_(ASM) × β_(REAR)) = −1.681

Among the values for each of the embodiments, r₁, r₂, . . . indicate radii of curvature for each lens, diaphragm surface, etc.; d₁, d₂, . . . indicate the thickness of the lenses, diaphragm, etc. or the distance between surfaces thereof; nd₁, nd₂, . . . indicate the refraction index of each lens with respect to the d-line (λ=587.56 nm); and υd₁, υd₂, . . . indicate the Abbe number with respect to the d-line (λ=587.56 nm) of each lens. Lengths are indicated in units of [mm] and angles are indicated in [degrees].

Each aspheric surface shape above is expressed by equation [1], where Z is the distance from the vertex of the lens surface to the optical axis; c(=1/r, where, r=radius of curvature) is paraxial radius of curvature at the vertex of the lens surface; h is the height from the optical axis; K is the constant of the cone; A₄ and A₆ are the fourth and sixth aspheric coefficients, respectively; and the travel direction of light is positive.

$\begin{matrix} {Z = {\frac{{ch}^{2}}{1 + \sqrt{1 - {\left( {1 + k} \right)c^{2}h^{2}}}} + {A_{4}h^{4}} + {A_{6}h^{6}}}} & \lbrack 1\rbrack \end{matrix}$

FIG. 2 is a diagram of various types of aberration (at infinity focus) in the imaging lens according to the first embodiment. In the diagram, g indicates the wavelength aberration corresponding to g-line (λ=435.83 nm), d indicates the wavelength aberration corresponding to d-line (λ=587.56 nm). Furthermore, in the diagram, S and M shown with respect to astigmatism, respectively indicate aberration at the sagittal image plane and at the meridonal image plane.

As described, in the imaging lens of the first embodiment, the focusing lens group and the anti-vibration lens group are respectively configured by 1 lens, facilitating reductions in the size and weight of these movable lens groups and enabling further reduction of the size of the optical system overall. In particular, satisfaction of the conditional expressions above suppresses the distance that the focusing lens group and the anti-vibration lens group move, facilitating further reduction of the size of the optical system and enabling further improvement of the imaging performance of the optical system. Further, the imaging lens of the first embodiment uses, as necessary, lenses having an aspheric surface and cemented lenses, whereby favorable imaging performance can be maintained by fewer lenses.

FIG. 3 is a cross sectional view (along the optical axis) of the imaging lens according to a second embodiment. The imaging lens includes sequentially from a side (object side) nearest a non-depicted object, a first lens group G₂₁ having a positive refractive power, a second lens group G₂₂ having a negative refractive power, a third lens group G₂₃ having a positive refractive power, and a fourth lens group G₂₄ having a positive refractive power. The iris diaphragm STO is disposed between the first lens group G₂₁ and the second lens group G₂₂. The iris diaphragm STO has multiple diaphragm blades and by arbitrarily changing the aperture, controls the incident optical ray bundle and adjusts the F number. Between the fourth lens group G₂₄ and the image plane IMG, the filter FL, the field stop FS, the filter FL, and the cover glass CG are disposed sequentially from the object side. The filters FL are disposed to block infrared light. The field stop FS is disposed to suppress optical ray bundles that are not related to the image and cause unnecessary internal reflection. The cover glass CG is disposed to protect the image plane IMG. The filters FL and the cover glass CG are disposed as needed and when not necessary, can be omitted. The optical receiving surface of the image sensor, such as a CCD, CMOS, etc. is disposed at the image plane IMG.

The first lens group G₂₁ includes, sequentially from the object side, a negative lens L₂₁₁, a positive lens L₂₁₂, a positive lens L₂₁₃, a negative lens L₂₁₄, a positive lens L₂₁₅, a negative lens L₂₁₆, and a positive lens L₂₁₇. The positive lens L₂₁₅ and the negative lens L₂₁₆ are cemented.

The second lens group G₂₂ is configured by a negative lens L₂₂₁.

The third lens group G₂₃ is configured by a positive lens L₂₃₁. The surface of the object side of the positive lens L₂₃₁ is aspheric. The third lens group G₂₃ is moved along the optical axis, from the image plane IMG side toward the object side, whereby focusing from the state of infinity focus to the focus state for the minimum object distance is performed.

The fourth lens group G₂₄ includes, sequentially from the object side, a front group G_(24F) having a negative refractive power, an intermediate group G_(24M) having a negative refractive power, the aperture stop AS, and a rear group G_(24R) having a positive refractive power. The aperture stop AS suppresses the occurrence of aberration, which causes deterioration of the imaging performance of the optical system. The front group G_(24F) includes, sequentially from the object side, a negative lens L₂₄₁ and a positive lens L₂₄₂. The negative lens L₂₄₁ and the positive lens L₂₄₂ are cemented. The intermediate group G_(24M) is configured by a negative lens L₂₄₃. Both surfaces of the negative lens L₂₄₃ are aspheric. The intermediate group G_(24M) has a function as the anti-vibration lens group. In other words, the intermediate group G_(24M) is moved in a direction that is substantially perpendicular to the optical axis (shifting of lens center), whereby blur that occurs when the optical system is subject to vibration consequent to handshake, is corrected. The rear group G_(24R) includes, sequentially from the object side, a negative lens L₂₄₄, a positive lens L₂₄₅, a positive lens L₂₄₆, and a negative lens L₂₄₇. The negative lens L₂₄₄ and the positive lens L₂₄₅ are cemented.

Various values related to the imaging lens according to the second embodiment are indicated below.

(Lens data) r₁ = 219.426 d₁ = 5.000 nd₁ = 1.51680 νd₁ = 64.2 r₂ = 134.759 d₂ = 0.500 r₃ = 101.237 d₃ = 19.623 nd₂ = 1.49700 νd₂ = 81.6 r₄ = −405.162 d₄ = 0.300 r₅ = 127.435 d₅ = 11.549 nd₃ = 1.49700 νd₃ = 81.6 r₆ = −3866.078 d₆ = 2.713 r₇ = −375.484 d₇ = 4.000 nd₄ = 1.51680 νd₄ = 64.2 r₈ = 67.794 d₈ = 4.283 r₉ = 85.387 d₉ = 16.051 nd₅ = 1.49700 νd₅ = 81.6 r₁₀ = −283.495 d₁₀ = 4.000 nd₆ = 1.80610 νd₆ = 33.3 r₁₁ = 311.150 d₁₁ = 0.200 r₁₂ = 102.482 d₁₂ = 7.098 nd₇ = 1.84666 νd₇ = 23.8 r₁₃ = 401.606 d₁₃ = 22.225 r₁₄ = ∞ (iris diaphragm) d₁₄ = 12.242 r₁₅ = 299.644 d₁₅ = 2.000 nd₈ = 1.84666 νd₈ = 23.8 r₁₆ = 63.960 d₁₆ = 30.324 r₁₇ = 89.122 (aspheric surface) d₁₇ = 4.000 nd₉ = 1.62263 νd₉ = 58.2 r₁₈ = 1359.164 d₁₈ = 5.000 r₁₉ = 119.166 d₁₉ = 1.700 nd₁₀ = 1.80610 νd₁₀ = 33.3 r₂₀ = 29.620 d₂₀ = 6.687 nd₁₁ = 1.48749 νd₁₁ = 70.4 r₂₁ = −416.013 d₂₁ = 4.267 r₂₂ = −90.410 (aspheric surface) d₂₂ = 2.000 nd₁₂ = 1.62263 νd₁₂ = 58.2 r₂₃ = 47.203 (aspheric surface) d₂₃ = 6.291 r₂₄ = ∞ (aperture stop) d₂₄ = 4.933 r₂₅ = 107.620 d₂₅ = 1.900 nd₁₃ = 1.84666 νd₁₃ = 23.8 r₂₆ = 36.734 d₂₆ = 8.410 nd₁₄ = 1.80610 νd₁₄ = 33.3 r₂₇ = −304.490 d₂₇ = 17.803 r₂₈ = 178.581 d₂₈ = 7.617 nd₁₅ = 1.75520 νd₁₅ = 27.5 r₂₉ = −65.112 d₂₉ = 7.685 r₃₀ = −65.282 d₃₀ = 1.600 nd₁₆ = 1.72916 νd₁₆ = 54.7 r₃₁ = −857.194 d₃₁ = 8.000 r₃₂ = ∞ d₃₂ = 2.000 nd₁₇ = 1.51680 νd₁₇ = 64.2 r₃₃ = ∞ d₃₃ = 8.000 r₃₄ = ∞ (field stop) d₃₄ = 39.800 r₃₅ = ∞ d₃₅ = 2.200 nd₁₈ = 1.51680 νd₁₈ = 64.2 r₃₆ = ∞ d₃₆ = 1.000 r₃₇ = ∞ d₃₇ = 1.000 nd₁₉ = 1.51680 νd₁₉ = 64.2 r₃₈ = ∞ d₃₈ = 1.000 r₃₉ = ∞ (image plane) (Constants of the cone (k) and aspheric coefficients (A₄, A₆) ) (Seventeenth plane) k = −8.38717 × 10⁻¹, A₄ = 2.02307 × 10⁻⁷, A₆ = 7.21561 × 10⁻¹¹ (Twenty-second plane) k = −1.44884, A₄ = 0, A₆ = 0 (Twenty-third plane) k = −1.89336, A₄ = 0, A₆ = 0 f (focal length of entire optical system) = 294.00 F no. = 2.88 2ω (angle of view) = 8.3 (Values related to conditional expression (1)) f₁/f = 0.444 (Values related to conditional expression (2)) |f₂|/f = 0.325 (Values related to conditional expression (3)) f₃/f = 0.518 (Values related to conditional expression (4)) |f₄|/f = 3.045 (Values related to conditional expression (5)) β₄ ² − (β₃ × (β₄)² = 1.717 (Values related to conditional expression (6)) f₁₋₂/f = 1.178 (Values related to conditional expression (7)) ν_(dA) (Abbe number for d-line of negative lens L₂₁₁, negative lens L₂₁₄) = 64.2 (Values related to conditional expression (8)) ν_(dB) (Abbe number for d-line of positive lens L₂₁₇) = 23.8 (Values related to conditional expression (9)) |f_(ASM)|/f = 0.168 (Values related to conditional expression (10)) f_(REAR)/f = 0.221 (Values related to conditional expression (11)) β_(REAR) − (β_(ASM) × β_(REAR)) = −1.676

Among the values for each of the embodiments, r₁, r₂, . . . indicate radii of curvature for each lens, diaphragm surface, etc.; d₁, d₂, . . . indicate the thickness of the lenses, diaphragm, etc. or the distance between surfaces thereof; nd₁, nd₂, . . . indicate the refraction index of each lens with respect to the d-line (λ=587.56 nm); and υd₁, υd₂, . . . indicate the Abbe number with respect to the d-line (λ=587.56 nm) of each lens. Lengths are indicated in units of [mm] and angles are indicated in [degrees].

Each aspheric surface shape above is expressed by equation [1], where Z is the distance from the vertex of the lens surface to the optical axis; c(=1/r, where, r=radius of curvature) is paraxial radius of curvature at the vertex of the lens surface; h is the height from the optical axis; K is the constant of the cone; A₄ and A₆ are the fourth and sixth aspheric coefficients, respectively; and the travel direction of light is positive.

FIG. 4 is a diagram of various types of aberration (at infinity focus) in the imaging lens according to the second embodiment. In the diagram, g indicates the wavelength aberration corresponding to g-line (λ=435.83 nm), d indicates the wavelength aberration corresponding to d-line (λ=587.56 nm). Furthermore, in the diagram, S and M shown with respect to astigmatism, respectively indicate aberration at the sagittal image plane and at the meridonal image plane.

As described, in the imaging lens of the second embodiment, the focusing lens group and the anti-vibration lens group are respectively configured by 1 lens, facilitating reductions in the size and weight of these movable lens groups and enabling further reduction of the size of the optical system overall. In particular, satisfaction of the conditional expressions above suppresses the distance that the focusing lens group and the anti-vibration lens group move, facilitating further reduction of the size of the optical system and enabling further improvement of the imaging performance of the optical system. Further, the imaging lens of the second embodiment uses, as necessary, lenses having an aspheric surface and cemented lenses, whereby favorable imaging performance can be maintained by fewer lenses.

FIG. 5 is a cross sectional view (along the optical axis) of the imaging lens according to a third embodiment. The imaging lens includes sequentially from a side (object side) nearest a non-depicted object, a first lens group G₃₁ having a positive refractive power, a second lens group G₃₂ having a negative refractive power, a third lens group G₃₃ having a positive refractive power, and a fourth lens group G₃₄ having a positive refractive power. The iris diaphragm STO is disposed between the first lens group G₃₁ and the second lens group G₃₂. The iris diaphragm STO has multiple diaphragm blades and by arbitrarily changing the aperture, controls the incident optical ray bundle and adjusts the F number. Between the fourth lens group G₃₄ and the image plane IMG, the filter FL, the field stop FS, the filter FL, and the cover glass CG are disposed sequentially from the object side. The filters FL are disposed to block infrared light. The field stop FS is disposed to suppress optical ray bundles that are not related to the image and cause unnecessary internal reflection. The cover glass CG is disposed to protect the image plane IMG. The filters FL and the cover glass CG are disposed as needed and when not necessary, can be omitted. The optical receiving surface of the image sensor, such as a CCD, CMOS, etc. is disposed at the image plane IMG.

The first lens group G₃₁ includes, sequentially from the object side, a negative lens L₃₁₁, a positive lens L₃₁₂, a positive lens L₃₁₃, a negative lens L₃₁₄, a positive lens L₃₁₅, a negative lens L₃₁₆, and a positive lens L₃₁₇. The positive lens L₃₁₅ and the negative lens L₃₁₆ are cemented.

The second lens group G₃₂ is configured by a negative lens L₃₂₁.

The third lens group G₃₃ is configured by a positive lens L₃₃₁. The surface of the object side of the positive lens L₃₃₁ is aspheric. The third lens group G₃₃ is moved along the optical axis, from the image plane IMG side toward the object side, whereby focusing from the state of infinity focus to the focus state for the minimum object distance is performed.

The fourth lens group G₃₄ includes, sequentially from the object side, a front group G_(34F) having a negative refractive power, an intermediate group G_(34M) having a negative refractive power, the aperture stop AS, and a rear group G_(34R) having a positive refractive power. The aperture stop AS suppresses the occurrence of aberration, which causes deterioration of the imaging performance of the optical system. The front group G_(34F) includes, sequentially from the object side, a negative lens L₃₄₁ and a positive lens L₃₄₂. The negative lens L₃₄₁ and the positive lens L₃₄₂ are cemented. The intermediate group G_(34M) is configured by a negative lens L₃₄₃. Both surfaces of the negative lens L₃₄₃ are aspheric. The intermediate group G_(34M) has a function as the anti-vibration lens group. In other words, the intermediate group G_(34M) is moved in a direction that is substantially perpendicular to the optical axis (shifting of lens center), whereby blur that occurs when the optical system is subject to vibration consequent to handshake, is corrected. The rear group G_(34R) includes, sequentially from the object side, a negative lens L₃₄₄, a positive lens L₃₄₅, a positive lens L₃₄₆, and a negative lens L₃₄₇. The negative lens L₃₄₄ and the positive lens L₃₄₅ are cemented.

Various values related to the imaging lens according to the third embodiment are indicated below.

(Lens data) r₁ = 220.178 d₁ = 5.000 nd₁ = 1.51680 νd₁ = 64.2 r₂ = 135.079 d₂ = 0.500 r₃ = 101.861 d₃ = 19.514 nd₂ = 1.49700 νd₂ = 81.6 r₄ = −421.241 d₄ = 0.313 r₅ = 135.294 d₅ = 11.021 nd₃ = 1.49700 νd₃ = 81.6 r₆ = −4074.120 d₆ = 2.701 r₇ = −382.643 d₇ = 4.000 nd₄ = 1.51680 νd₄ = 64.2 r₈ = 69.080 d₈ = 4.054 r₉ = 85.435 d₉ = 16.480 nd₅ = 1.49700 νd₅ = 81.6 r₁₀ = −275.253 d₁₀ = 4.000 nd₆ = 1.80610 νd₆ = 33.3 r₁₁ = 325.486 d₁₁ = 0.200 r₁₂ = 106.114 d₁₂ = 7.163 nd₇ = 1.84666 νd₇ = 23.8 r₁₃ = 443.488 d₁₃ = 23.592 r₁₄ = ∞ (iris diaphragm) d₁₄ = 15.087 r₁₅ = 372.717 d₁₅ = 2.000 nd₈ = 1.84666 νd₈ = 23.8 r₁₆ = 64.213 d₁₆ = 28.151 r₁₇ = 86.524 (aspheric surface) d₁₇ = 4.000 nd₉ = 1.67790 νd₉ = 54.9 r₁₈ = 1877.282 d₁₈ = 5.000 r₁₉ = 126.348 d₁₉ = 1.700 nd₁₀ = 1.80000 νd₁₀ = 29.9 r₂₀ = 30.389 d₂₀ = 6.382 nd₁₁ = 1.48749 νd₁₁ = 70.4 r₂₁ = −381.691 d₂₁ = 4.077 r₂₂ = −104.099 (aspheric surface) d₂₂ = 2.000 nd₁₂ = 1.67790 νd₁₂ = 54.9 r₂₃ = 48.888 (aspheric surface) d₂₃ = 6.850 r₂₄ = ∞ (aperture stop) d₂₄ = 6.029 r₂₅ = 96.499 d₂₅ = 2.500 nd₁₃ = 1.84666 νd₁₃ = 23.8 r₂₆ = 38.478 d₂₆ = 8.537 nd₁₄ = 1.80610 νd₁₄ = 33.3 r₂₇ = −305.357 d₂₇ = 15.280 r₂₈ = 233.243 d₂₈ = 7.757 nd₁₅ = 1.80518 νd₁₅ = 25.5 r₂₉ = −65.129 d₂₉ = 4.920 r₃₀ = −62.149 d₃₀ = 1.643 nd₁₆ = 1.72916 νd₁₆ = 54.7 r₃₁ = −1262.573 d₃₁ = 8.000 r₃₂ = ∞ d₃₂ = 2.000 nd₁₇ = 1.51680 νd₁₇ = 64.2 r₃₃ = ∞ d₃₃ = 8.000 r₃₄ = ∞ (field stop) d₃₄ = 41.348 r₃₅ = ∞ d₃₅ = 2.200 nd₁₈ = 1.51680 νd₁₈ = 64.2 r₃₆ = ∞ d₃₆ = 1.000 r₃₇ = ∞ d₃₇ = 1.000 nd₁₉ = 1.51680 νd₁₉ = 64.2 r₃₈ = ∞ d₃₈ = 1.000 r₃₉ = ∞ (image plane) (Constants of the cone (k) and aspheric coefficients (A₄, A₆)) (Seventeenth plane) k = −2.93110 × 10⁻¹, A₄ = 9.64053 × 10⁻⁸, A₆ = 6.73961 × 10⁻¹¹ (Twenty-second plane) k = −1.80583, A₄ = 0, A₆ = 0 (Twenty-third plane) k = −1.78076, A₄ = 0, A₆ = 0 f (focal length of entire optical system) = 294.00 F no. = 2.88 2ω (angle of view) = 8.3 (Values related to conditional expression (1)) f₁/f = 0.453 (Values related to conditional expression (2)) |f₂|/f = 0.310 (Values related to conditional expression (3)) f₃/f = 0.453 (Values related to conditional expression (4)) |f₄|/f = 2.577 (Values related to conditional expression (5)) β₄ ² − (β₃ × β₄)² = 1.919 (Values related to conditional expression (6)) f₁₋₂/f = 1.273 (Values related to conditional expression (7)) ν_(dA) (Abbe number for d-line of negative lens L₃₁₁, negative lens L₃₁₄) = 64.2 (Values related to conditional expression (8)) ν_(dB) (Abbe number for d-line of positive lens L₃₁₇) = 23.8 (Values related to conditional expression (9)) |f_(ASM)|/f = 0.165 (Values related to conditional expression (10)) f_(REAR)/f = 0.224 (Values related to conditional expression (11)) β_(REAR) − (β_(ASM) × β_(REAR)) = −1.680

Among the values for each of the embodiments, r₁, r₂, . . . indicate radii of curvature for each lens, diaphragm surface, etc.; d₁, d₂, . . . indicate the thickness of the lenses, diaphragm, etc. or the distance between surfaces thereof; nd₁, nd₂, . . . indicate the refraction index of each lens with respect to the d-line (λ=587.56 nm); and υd₁, υd₂, . . . indicate the Abbe number with respect to the d-line (λ=587.56 nm) of each lens. Lengths are indicated in units of [mm] and angles are indicated in [degrees].

Each aspheric surface shape above is expressed by equation [1], where Z is the distance from the vertex of the lens surface to the optical axis; c(=1/r, where, r=radius of curvature) is paraxial radius of curvature at the vertex of the lens surface; h is the height from the optical axis; K is the constant of the cone; A₄ and A₆ are the fourth and sixth aspheric coefficients, respectively; and the travel direction of light is positive.

FIG. 6 is a diagram of various types of aberration (at infinity focus) in the imaging lens according to the third embodiment. In the diagram, g indicates the wavelength aberration corresponding to g-line (λ=435.83 nm), d indicates the wavelength aberration corresponding to d-line (λ=587.56 nm). Furthermore, in the diagram, S and M shown with respect to astigmatism, respectively indicate aberration at the sagittal image plane and at the meridonal image plane.

As described, in the imaging lens of the third embodiment, the focusing lens group and the anti-vibration lens group are respectively configured by 1 lens, facilitating reductions in the size and weight of these movable lens groups and enabling further reduction of the size of the optical system overall. In particular, satisfaction of the conditional expressions above suppresses the distance that the focusing lens group and the anti-vibration lens group move, facilitating further reduction of the size of the optical system and enabling further improvement of the imaging performance of the optical system. Further, the imaging lens of the third embodiment uses, as necessary, lenses having an aspheric surface and cemented lenses, whereby favorable imaging performance can be maintained by fewer lenses.

FIG. 7 is a cross sectional view (along the optical axis) of the imaging lens according to a fourth embodiment. The imaging lens includes sequentially from a side (object side) nearest a non-depicted object, a first lens group G₄₁ having a positive refractive power, a second lens group G₄₂ having a negative refractive power, a third lens group G₄₃ having a positive refractive power, and a fourth lens group G₄₄ having a positive refractive power. The iris diaphragm STO is disposed between the first lens group G₄₁ and the second lens group G₄₂. The iris diaphragm STO has multiple diaphragm blades and by arbitrarily changing the aperture, controls the incident optical ray bundle and adjusts the F number. The aperture stop AS is disposed between the third lens group G₄₃ and the fourth lens group G₄₄. The aperture stop AS suppresses the occurrence of aberration, which causes deterioration of the imaging performance of the optical system. Between the fourth lens group G₄₄ and the image plane IMG, the filter FL, the field stop FS, the filter FL, and the cover glass CG are disposed sequentially from the object side. The filters FL are disposed to block infrared light. The field stop FS is disposed to suppress optical ray bundles that are not related to the image and cause unnecessary internal reflection. The cover glass CG is disposed to protect the image plane IMG. The filters FL and the cover glass CG are disposed as needed and when not necessary, can be omitted. The optical receiving surface of the image sensor, such as a CCD, CMOS, etc. is disposed at the image plane IMG.

The first lens group G₄₁ includes, sequentially from the object side, a negative lens L₄₁₁, a positive lens L₄₁₂, a positive lens L₄₁₃, a negative lens L₄₁₄, a positive lens L₄₁₅, a negative lens L₄₁₆, and a positive lens L₄₁₇. The positive lens L₄₁₅ and the negative lens L₄₁₆ are cemented.

The second lens group G₄₂ is configured by a negative lens L₄₂₁.

The third lens group G₄₃ is configured by a positive lens L₄₃₁. The third lens group G₄₃ is moved along the optical axis, from the image plane IMG side toward the object side, whereby focusing from the state of infinity focus to the focus state for the minimum object distance is performed.

The fourth lens group G₄₄ includes, sequentially from the object side, a front group G_(44F) having a negative refractive power and a rear group G_(44R) having a positive refractive power. The front group G_(44F) is configured by a negative lens L₄₄₁. Both surfaces of the negative lens L₄₄₁ are aspheric. The front group G_(44F) has a function as the anti-vibration lens group. In other words, the front group G_(44F) is moved in a direction that is substantially perpendicular to the optical axis (shifting of lens center), whereby blur that occurs when the optical system is subject to vibration consequent to handshake, is corrected. The rear group G_(44R) includes, sequentially from the object side, a negative lens L₄₄₂, a positive lens L₄₄₃, a positive lens L₄₄₄, and a negative lens L₄₄₅. The negative lens L₄₄₂ and the positive lens L₄₄₃ are cemented. Furthermore, the positive lens L₄₄₄ and the negative lens L₄₄₅ are cemented.

Various values related to the imaging lens according to the fourth embodiment are indicated below.

(Lens data) r₁ = 231.617 d₁ = 5.000 nd₁ = 1.51680 νd₁ = 64.2 r₂ = 130.102 d₂ = 1.260 r₃ = 100.245 d₃ = 21.011 nd₂ = 1.49700 νd₂ = 81.6 r₄ = −301.902 d₄ = 0.996 r₅ = 139.863 d₅ = 10.876 nd₃ = 1.49700 νd₃ = 81.6 r₆ = −1782.771 d₆ = 3.192 r₇ = −290.488 d₇ = 4.000 nd₄ = 1.51680 νd₄ = 64.2 r₈ = 63.286 d₈ = 2.602 r₉ = 67.471 d₉ = 19.975 nd₅ = 1.49700 νd₅ = 81.6 r₁₀ = −239.203 d₁₀ = 4.000 nd₆ = 1.90366 νd₆ = 31.3 r₁₁ = 330.756 d₁₁ = 0.200 r₁₂ = 92.906 d₁₂ = 7.409 nd₇ = 1.84666 νd₇ = 23.8 r₁₃ = 340.354 d₁₃ = 28.096 r₁₄ = ∞ (iris diaphragm) d₁₄ = 3.399 r₁₅ = 194.065 d₁₅ = 2.000 nd₈ = 1.80518 νd₈ = 25.5 r₁₆ = 53.052 d₁₆ = 29.037 r₁₇ = 83.952 d₁₇ = 4.000 nd₉ = 1.61800 νd₉ = 63.4 r₁₈ = 574.211 d₁₈ = 5.279 r₁₉ = ∞ (aperture stop) d₁₉ = 3.336 r₂₀ = −93.669 (aspheric surface) d₂₀ = 2.058 nd₁₀ = 1.61881 νd₁₀ = 63.9 r₂₁ = 49.638 (aspheric surface) d₂₁ = 5.441 r₂₂ = 266.974 d₂₂ = 1.500 nd₁₁ = 1.84666 νd₁₁ = 23.8 r₂₃ = 54.721 d₂₃ = 4.783 nd₁₂ = 1.62299 νd₁₂ = 58.1 r₂₄ = −121.377 d₂₄ = 24.477 r₂₅ = 125.164 d₂₅ = 9.417 nd₁₃ = 1.69895 νd₁₃ = 30.1 r₂₆ = −48. 649 d₂₆ = 2.500 nd₁₄ = 1.60562 νd₁₄ = 43.7 r₂₇ = −341.764 d₂₇ = 10.912 r₂₈ = ∞ d₂₈ = 2.000 nd₁₅ = 1.51680 νd₁₅ = 64.2 r₂₉ = ∞ d₂₉ = 10.912 r₃₀ = ∞ (field stop) d₃₀ = 50.131 r₃₁ = ∞ d₃₁ = 2.200 nd₁₆ = 1.51680 νd₁₆ = 64.2 r₃₂ = ∞ d₃₂ = 1.000 r₃₃ = ∞ d₃₃ = 1.000 nd₁₇ = 1.51680 νd₁₇ = 64.2 r₃₄ = ∞ d₃₄ = 1.000 r₃₅ = ∞ (image plane) (Constants of the cone (k) and aspheric coefficients (A₄, A₆)) (Twentieth plane) k = −3.74645, A₄ = 0, A₆ = 0 (Twenty-first plane) k = −1.65975, A₄ = 0, A₆ = 0 f (focal length of entire optical system) = 294.00 F no. = 2.88 2ω (angle of view) = 8. 3 (Values related to conditional expression (1)) f₁/f = 0.429 (Values related to conditional expression (2)) |f₂|/f = 0.308 (Values related to conditional expression (3) f₃/f = 0.537 (Values related to conditional expression (4)) |f₄|/f = 15.090 (Values related to conditional expression (5)) β₄ ² − (β₃ × β₄)² = 1.639 (Values related to conditional expression (6)) f₁₋₂/f = 1.137 (Values related to conditional expression (7)) ν_(dA) (Abbe number for d-line of negative lens L₄₁₁, negative lens L₄₁₄) = 64.2 (Values related to conditional expression (8)) ν_(dB) (Abbe number for d-line of positive lens L₄₁₇) = 23.8 (Values related to conditional expression (10)) f_(REAR)/f = 0.275 (Values related to conditional expression (12)) |f_(ASF)|/f = 0.177 (Values related to conditional expression (13)) β_(REAR) − (β_(ASF) × β_(REAR)) = −1.681

Among the values for each of the embodiments, r₁, r₂, . . . indicate radii of curvature for each lens, diaphragm surface, etc.; d₁, d₂, . . . indicate the thickness of the lenses, diaphragm, etc. or the distance between surfaces thereof; nd₁, nd₂, . . . indicate the refraction index of each lens with respect to the d-line (λ=587.56 nm); and υd₁, υd₂, . . . indicate the Abbe number with respect to the d-line (λ=587.56 nm) of each lens. Lengths are indicated in units of [mm] and angles are indicated in [degrees].

Each aspheric surface shape above is expressed by equation [1], where Z is the distance from the vertex of the lens surface to the optical axis; c(=1/r, where, r=radius of curvature) is paraxial radius of curvature at the vertex of the lens surface; h is the height from the optical axis; K is the constant of the cone; A₄ and A₆ are the fourth and sixth aspheric coefficients, respectively; and the travel direction of light is positive.

FIG. 8 is a diagram of various types of aberration (at infinity focus) in the imaging lens according to the fourth embodiment. In the diagram, g indicates the wavelength aberration corresponding to g-line (λ=435.83 nm), d indicates the wavelength aberration corresponding to d-line (λ=587.56 nm). Furthermore, in the diagram, S and M shown with respect to astigmatism, respectively indicate aberration at the sagittal image plane and at the meridonal image plane.

As described in the imaging lens of the fourth embodiment, the focusing lens group and the anti-vibration lens group are respectively configured by 1 lens, facilitating reductions in the size and weight of these movable lens groups and enabling further reduction of the size of the optical system overall. In particular, satisfaction of the conditional expressions above suppresses the distance that the focusing lens group and the anti-vibration lens group move, facilitating further reduction of the size of the optical system and enabling further improvement of the imaging performance of the optical system. In particular, in the imaging lens of the fourth embodiment, the fourth lens group, which includes the anti-vibration lens group, is configured by 2 lens groups, further simplifying the configuration of the optical system overall. The imaging lens of the fourth embodiment uses, as necessary, lenses having an aspheric surface and cemented lenses, whereby favorable imaging performance can be maintained by fewer lenses.

Furthermore, according to the present invention, configuration of the front group by a single lens element enables effective suppression of aberration at risk of occurring.

As described, the imaging lens according to the present invention is useful for digital cameras, video cameras, etc. and in particular, is suitable for imaging apparatuses equipped with a contrast detection autofocus mechanism.

Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.

The present document incorporates by reference the entire contents of Japanese priority document, 2011-110768 and 2011-110769 filed in Japan on May 17, 2011. 

What is claimed is:
 1. An imaging lens comprising, sequentially from an object side: a first lens group having a positive refractive power; a second lens group having a negative refractive power; a third lens group having a positive refractive power; and a fourth lens group having a negative refractive power, wherein the third lens group is configured by a single lens element, and the third lens group is moved along an optical axis, whereby focusing is performed, wherein the following conditional expressions are satisfied: 0.4<f ₁ /f<0.5,   (1) 0.2<|f ₂ |f<0.4,   (2) 0.4<f ₃ /f<0.6, and   (3) 2.0<|f ₄ |/f<16.0,   (4) wherein f is the focal length of the entire optical system, f₁ is the focal length of the first lens group, f₂ is the focal length of the second lens group, f₃ is the focal length of the third lens group, and f₄ is the focal length of the fourth lens group.
 2. The imaging lens according to claim 1, wherein a conditional expression (5) 1.5<β₄ ²−(β₃×β₄)²<2.5 is satisfied; wherein β₃ is lateral magnification of the third lens group and β₄ is lateral magnification of the fourth lens group.
 3. The imaging lens according to claim 1, wherein a conditional expression (6) 1.0<f₁₋₂/f<2.0 is satisfied; wherein f₁₋₂ is the combined focal length of the first lens group and the second lens group and f is the focal length of the entire optical system.
 4. The imaging lens according to claim 1, wherein the first lens group includes at least one negative lens that satisfies a conditional expression (7) υ_(dA)>60; wherein υ_(dA) is the Abbe number for the d-line of at least one negative lens.
 5. The imaging lens according to claim 1, further comprising a positive lens that is among the first lens group and is disposed closest to the image and satisfies a conditional expression (8) υ_(dB)<35; wherein υ_(dB) is the Abbe number for the d-line of the positive lens.
 6. The imaging lens according to claim 1, wherein the fourth lens group includes, sequentially from the object side: a front group having a negative refractive power, an intermediate group having a negative refractive power, and a rear group having a positive refractive power, the intermediate group is configured by a single lens element, and the intermediate group is configured to move in a direction substantially perpendicular to the optical axis so as to correct blur that occurs when the optical system is subject to vibration.
 7. The imaging lens according to claim 6, wherein the following conditional expressions are satisfied: 0.1<|f _(ASM) |/f<0.2 and   (9) 0.2<f _(REAR) /f<0.3;   (10) wherein f_(ASM) is the focal length of the intermediate group, f_(REAR) is the focal length of the rear group, and f is the focal length of the entire optical system.
 8. The imaging lens according to claim 6, wherein a conditional expression (11) −2.0<β_(REAR)−(β_(ASM)×β_(REAR))<−1.6 is satisfied; wherein β_(REAR) is lateral magnification of the rear group and β_(ASM) is lateral magnification of the intermediate group.
 9. The imaging lens according to claim 1, wherein the fourth lens group includes, sequentially from the object side: a front group having a negative refractive power and a rear group having a positive refractive power, the front group is configured by a single lens element, and the front group is configured to move in a direction substantially perpendicular to the optical axis so as to correct blur that occurs when the optical system is subject to vibration.
 10. The imaging lens according to claim 9, wherein the following conditional expressions are satisfied: 0.1<|f _(ASF) |/f<0.2, and   (12) 0.2<f _(REAR) /f<0.3;   (10) wherein f_(ASF) is the focal length of the front group, f_(REAR) is the focal length of the rear group, and f is the focal length of the entire optical system.
 11. The imaging lens according to claim 9, wherein a conditional expression (13) −2.0<β_(REAR)−(β_(ASF)×β_(REAR))<−1.6 is satisfied; wherein β_(REAR) is lateral magnification of the rear group and β_(ASF) is lateral magnification of the front group.
 12. An imaging lens comprising, sequentially from an object side: a first lens group having a positive refractive power; a second lens group having a negative refractive power; a third lens group having a positive refractive power; and a fourth lens group having a negative refractive power, wherein the fourth lens group includes, sequentially from the object side: a front group having a negative refractive power, an intermediate group having a negative refractive power, and a rear group having a positive refractive power, the intermediate group is configured by a single lens element, and the intermediate group is configured to move in a direction substantially perpendicular to the optical axis so as to correct blur that occurs when the optical system is subject to vibration.
 13. The imaging lens according to claim 12, wherein the following conditional expressions are satisfied: 0.1<|f _(ASM) |/f<0.2, and   (9) 0.2<f _(REAR) /f<0.3;   (10) wherein f_(ASM) is the focal length of the intermediate group, f_(REAR) is the focal length of the rear group, and f is the focal length of the entire optical system.
 14. The imaging lens according to claim 12, wherein a conditional expression (11) −2.0<β_(REAR)−(β_(ASM)×β_(REAR))<−1.6 is satisfied; wherein β_(REAR) is lateral magnification of the rear group and β_(ASM) is lateral magnification of the intermediate group.
 15. The imaging lens according to claim 12, wherein the intermediate group has at least one aspheric surface.
 16. An imaging lens comprising, sequentially from an object side: a first lens group having a positive refractive power; a second lens group having a negative refractive power; a third lens group having a positive refractive power; and a fourth lens group having a negative refractive power, wherein the fourth lens group includes, sequentially from the object side: a front group having a negative refractive power and a rear group having a positive refractive power, the front group is configured by a single lens element, and the front group is configured to move in a direction substantially perpendicular to the optical axis so as to correct blur that occurs when the optical system is subject to vibration.
 17. The imaging lens according to claim 16, wherein the following conditional expressions are satisfied: 0.1<|f _(ASF) |/f<0.2 and   (12) 0.2<f _(REAR) /f<0.3;   (10) wherein f_(ASF) is the focal length of the front group, f_(REAR) is the focal length of the rear group, and f is the focal length of the entire optical system.
 18. The imaging lens according to claim 16, wherein a conditional expression (13) −2.0<β_(REAR)−(β_(ASF)×β_(REAR))<−1.6 is satisfied; wherein β_(REAR) is lateral magnification of the rear group and β_(ASF) is lateral magnification of the front group.
 19. The imaging lens according to claim 16, wherein the front group has at least one aspheric surface. 